This invention relates generally to systems for dispensing radioactive materials in liquid form, and, more particularly, to hypodermic devices for injecting radioactive materials into a patient, usually for diagnostic purposes.
Radiopharmaceuticals utilized for such purposes emit highly penetrating gamma rays. The flow and distribution of these injected radiopharmaceuticals within organs and blood vessels are imaged by electronic gamma cameras. Although the harmful effects of radiation from a single such procedure on a patient are minimal, the cumulative effects of radiation on personnel involved in the preparation and administration of radiopharmaceuticals poses a significant hazard, widely believed to be the principal occupational hazard in diagnostic nuclear medicine. The degree of exposure is sufficiently high that the federal government has authorized the setting of occupational radiation levels ten times higher than those set for the general population. Significantly, studies have shown that over ninety percent of this radiation exposure is received during the handling of syringes used in administering radiopharmaceuticals.
Syringe shields of various designs have been used to provide some degree of protection, but have not been universally accepted. A syringe shield is typically a thick tube, made of lead or tungsten, that surrounds the barrel of the syringe. A narrow leaded glass port in the shield is often included, to permit viewing of the syringe contents. Although the radiation protection benefits of syringe shields are well known, such devices are still not widely accepted for a number of practical reasons relating to the injection process. Most injections are given by venipuncture of a major surface vein in the arm, and the tactile sensation as the hypodermic needle "pops" into a blood vessel is the principal indicator of proper venipuncture. Verification of proper entry into the vein can be obtained if blood is withdrawn when the syringe plunger is pulled back slightly. However, a disproportionately high percentage of patients subjected to nuclear medicine studies have blood vessels that are exceedingly difficult to enter, either because of repeated punctures for conventional medical injections, or because of health-related vascular deterioration. For these patients, the injection process, which for most normal persons might take about fifteen seconds, can easily take as long as a minute.
The difficulty encountered in inserting a hypodermic needle for purposes of a nuclear study is compounded by problems inherent in the use of a shielded syringe. Unfortunately, the weight and bulk of a shielded syringe substantially detract from the tactile sensation needed for proper venipuncture. Moreover, the thick wall of the shield necessitates injection at a substantially steeper angle than would otherwise be appropriate and desirable, and the needle is, therefore, more likely to penetrate the far wall of the blood vessel. The use of a syringe shield also results in a higher incidence of extravasation, i.e., injection into tissue outside the blood vessel. In addition to these problems, the reduced manipulative ability inherent in the use of a syringe shield increases the time required for proper venipuncture, and thereby reduces, to some degree, the benefits derived from the shielding.
Another drawback relating to syringe shields is that they are effective only when used with radiopharmaceuticals emitting low-energy gamma rays. When radiopharmaceuticals emitting high-energy gamma rays are used, the radiation passes virtually unchecked through the shield, resulting in substantially more radiation exposure. A number of important radiopharmaceuticals currently in use emit radiation at these relatively high energy levels. Many imaging studies must be completed within minutes after injection, and even those studies that require the acquisition of subsequent images, to examine metabolic utilization of the injected material, rarely require images more than several hours after injection. Consequently, the high-energy, short-lived radiopharmaceuticals are ideal for most imaging applications. The use of short-lived isotopes has permitted the dose of the injected radiopharmaceutical to be drastically increased, thereby improving the image quality without increasing the total patient radiation exposure. However, the radiation exposure to personnel who handle the syringes on a continuing basis is proportionately increased. Many of the short-lived radiopharmaceuticals used are positron emitters that are so energetic that a conventional syringe shield is rendered almost totally ineffective.
Another important aspect of nuclear imaging relates to the precise manner in which a radiopharmaceutical is injected. In obtaining images of various organs, it is often critical to obtain information concerning the hemodynamic flow patterns in the organs during the first minute or so following the injection. This technique, known as nuclear angiography, can be employed only if the radiopharmaceutical is injected in a compact concentration, known as a bolus. Unfortunately, an intravenously injected fluid normally tends to diffuse substantially before reaching the imaged organ, thereby resulting in a pronounced loss of sharpness in the resulting nuclear angiogram. Sometimes tourniquets and pressure cuffs are used on the upper arm in an attempt to hold the radioisotope in a bolus until the syringe contents have been completely injected.
In another technique, pre- and post-injections of saline are used to surround the radioisotope as it travels through the blood vessels. The principal diffusion then occurs between the saline and the blood, leaving the centrally located radioisotope in a relatively tight bolus. The saline technique is not widely used, however, because it requires multiple syringes, and a valve to switch the needle from one syringe to another. Moreover, the procedure for injecting saline before and after the radiopharmaceutical is necessarily more awkward, and demands more skill than a conventional injection.
It will be appreciated from the foregoing that there is a significant problem in injecting radiopharmaceutical materials into patients without exposing the personnel preparing and administering the materials to a substantial risk of radiation exposure. While syringe shields provide some degree of protection from low energy radioisotopes, at the cost of impaired facility of injection, there is essentially no technique available to protect personnel from radiation from the short-lived, higher energy radioisotopes. Ideally, what is required is a hypodermic device that retains the manipulative facility of an unshielded syringe, and yet eliminates substantially all radiation exposure in the procedures for preparing and injecting the materials. The present invention satisfies this ideal.